In order for rolling element bearings to operate in a reliable way, they have to be adequately lubricated. The main purpose of the lubricant is to prevent metallic contact between rolling elements, raceways and cages and thus prevent wear of the bearing. The lubricant is also used to protect the bearing surfaces against corrosion. It is therefore important to choose both the proper lubricant and the proper lubrication method for each individual bearing application, as well a correct maintenance program.
In applications where the lubrication for a bearing or bearings needs to be replenished or replaced, this can be done on the basis of relubrication intervals prescribed by the bearing manufacturer or the supplier of a lubrication system for the bearings. A generous safety factor is built into the prescribed intervals, but when, for example, extreme contamination enters a circulating lubricant, the prescribed interval may be too short. In other instances, the prescribed intervals can be excessively long, leading to the replacement of lubricant that still has a considerable useful life.
In the case of bearings in large machinery which are lubricated by a circulation lubrication system, for example, samples of the circulating oil can be taken on a regular basis to determine when an oil change is needed. One way making this determination is to measure the dielectric constant of the oil sample. It has been found that the dielectric constant of an oil changes as the oil degrades and becomes contaminated. And as an oil degrades, it loses its ability to effectively separate the rolling contact surfaces in a bearing.
Methods that involve taking oil samples to determine the actual condition of the lubricant have disadvantages. It is necessary to interfere with the lubrication system and the condition of the lubricant cannot be monitored continuously. Furthermore, a deterioration in the lubrication condition of a bearing need not be due to a deterioration in the condition of the lubricant. If, for example, an oil filter becomes clogged, a bearing might experience a starved lubrication condition simply because its supply of oil is interrupted. It is therefore advantageous to monitor lubrication condition.
In one known method for monitoring the lubrication condition of a bearing, the film thickness of the lubricant between the rolling elements and the bearing raceways is measured by measuring the capacitance between the bearing inner and outer ring. One such system is the LubCheck™ system of SKF. In this method, the bearing has to be electrically isolated from the electrical earth, i.e. the machine frame in which the bearing is mounted. Such a system is thus mainly suitable for laboratory testing of bearings. Also, the measurement method interferes with normal bearing operation.
Another way of estimating the lubricant film thickness is to measure the reflection of ultrasound from the oil layer in a bearing. Assuming the oil properties are known, the oil film thickness can be calculated from the reflection values. Again, this is an intrusive method that can be detrimental to bearing operation.
Consequently, there is a need for method of monitoring the lubrication condition in a bearing that is suitable for integration in an industrial environment and that can be implemented on-line, without affecting the normal operation of the bearing.